1. Field of the Invention
The present invention relates to a platinum temperature sensor and a method for producing the same, and in particular to a platinum temperature sensor in the case of which a platinum thin-film resistor, which is applied to a ceramic substrate, is used for temperature detection.
2. Description of Prior Art
A known platinum temperature sensor is shown in FIG. 3. In this known platinum temperature sensor, a platinum thin-film resistor 2 is applied to a ceramic substrate 4 which normally consists of aluminum oxide Al2O3. In the area in which the platinum thin-film resistor 2 is formed, a protective glaze 6 is provided on the surface of the ceramic substrate 4. The platinum layer, in which the platinum thin-film resistor 2 is normally formed in a meandering shape, is additionally patterned so as to include connecting areas 8 having lead wires 10 connected thereto in an electrically conductive manner for taking the sensor signal. For fixing the lead wires 10, a glaze 12 is provided.
The field of use of the platinum temperature sensor, which is shown in FIG. 3 and which is implemented in thin-film technology, is normally limited to 600° C. In the last few years, there has, however, been an increasing demand for an embodiment that can be used for higher operating temperatures, which may exceed 1,000° C. In the field of high-temperature sensors considerable efforts have therefore been made to provide platinum temperature sensors which are suitable to be used in such high temperature ranges. By purposefully selecting the composition of the protective glaze 6, it has already been possible to find satisfactory solutions for some cases of use, whereas in very particular fields of application, e.g. in special cases of use in the field of automotive engineering, the results do not satisfy all requirements. For example, the long-term stability of temperature sensors of the type described hereinbefore, especially when they have applied thereto a certain measurement current, which may e.g. be 5 mA, is not sufficiently guaranteed at the high temperatures occurring, viz. temperatures in the range of 800° C. and 1,000° C., since the protective glazes used may be decomposed electrochemically by the necessary measurement current at these high temperatures. The resultant material migration has a negative influence on the properties of the platinum so that the sta-bility of the sensors and, consequently, the measuring accuracy are impaired.
By purposefully selecting the composition of the protective glazes, improvements could be achieved to a certain extent, but it was impossible to find protective glazes that withstand the electrochemical decomposition by the measurement current in the case of continuous loads in a temperature range of 1,000° C. or more than 1,000° C.
From the article “Fügen von Technischen Keramiken mit Keramik-Grünfolien” by M. Neuhauser et al., sfi/Ber. DKG 72 (1995) Nr. 1–2, methods for joining technical ceramics are known wherein ceramics green foils are used to connect two ceramic layers. A prerequisite for the joining method described there is that the sintering temperature of the ce-ramic green foil is below the sintering temperature of the ceramic to be jointed.
A temperature sensor having a platinum resistance layer, which is applied to a ceramic substrate and encapsulated by a glaze, is disclosed in DE 7629727 U1.
From De 37 33 192 C1 a PTC-temperature sensor is known wherein a platinum resistor formed by means of a platinum thick-film technique is arranged between two ceramic green foils and an interlaminar binder layer, whereupon the two foils are laminated together by use of a pressure and a risen temperature and are sintered after that.
In DE 4445243 A1 a temperature sensor is described wherein three unprocessed ceramic substrates are laminated together, pressed and fired at 1.600° C. in order to form a uniform piece. Before laminating a platinum resistor is arranged between two of the ceramic substrates.